This chapter discusses regulatory uncertainty and change by focusing on a major legal and political disruptive force, that is Brexit. The chapter investigates how Brexit is affecting the conservation of marine biodiversity as well as the legitimacy and accountability of marine conservation regulation, thereby potentially impacting on the commoning of conservation law. The chapter, after a note to remind the reader of the multiple entanglements between EU and UK law, introduces key challenges to environmental law due to the changing governance architecture, considering specifically the role of environmental principles as well as that of institutions for ensuring the accountability for environmental standards. A section on the changes brought about by Brexit amendments to the Habitats Regulations follows and precedes the chapter conclusions.